Extreme large-scale atmospheric circulation associated with the “21·7” Henan flood

From July 19 to 21, 2021, Henan, a province in northern China(NC), was affected by severe flooding(referred to hereafter as “21·7”) caused by a prolonged record-breaking extreme precipitation(EP) event. Understanding the extremes of the large-scale circulation pattern during “21·7” is essential for predicting EP events and preventing future disasters. In this study, daily atmospheric large-scale circulations over NC in the summers from 1979 to 2021 were investigated using the circulation classification method of an obliquely rotated principal component analysis in T-mode(PCT). The geopotential heights at 500 hPa and 925 hPa were applied successively in classification. Among the nine summer circulation patterns found at 500 hPa, the three days of “21·7” belonged to the Type 8 pattern, which had the second highest probability of EP days among all patterns. It was characterized by a southeasterly wind toward North China Plain driven by a dipole geopotential height field, with the West Pacific subtropical high(WPSH) extending far north to 30°N and low pressure to the south near NC. Tropical cyclones(TCs) occurred on 72.5% of EP days, in which larger amounts of precipitation and a longer duration of EP days were found along the mountains in NC, as compared with other patterns. The distribution of EP events under this pattern was mainly influenced by the location of the low pressure at 925 hPa in the dipole. The subtype 8-3 circulation, with low pressure in the east of Taiwan Island, included “21·7” and accounted for 1.6% of all summer days. Typhoon In-fa, together with the WPSH, gave rise to intense column integrated moisture flux convergence(IMFC) via the southeasterly wind to Henan, which occurred continuously during the 3 days of “21·7”, resulting in the largest(second largest) mean IMFC among 3 consecutive EP days under type 8(all types) during the past 43 summers in NC.Further analysis revealed that the large-scale dynamic process could not completely explain the record-breaking EP during “21·7”,indicating possible contributions of other dynamic processes related to meso-scale convective storms.